1. Field of the Invention
The present invention relates to a method of forming a color image using a color scrolling technique and a projection system adopting this method, and more particularly, to a projection system and a method of forming a color image, in which a black bar is formed between color bars formed on a light valve, and a rising time and a falling time required to change image signals for the color bars are secured by controlling the width of the black bar.
2. Description of the Related Art
Generally, projection systems are classified into either 3-panel projection systems or single-panel projection systems, according to the number of light valves used. The light valves control the on/off operation of light on a pixel-by-pixel basis, thereby forming a picture. A high-output lamp is used as a light source. Single-panel projection systems can have smaller optical systems than can three-panel projection systems. However, these single-panel projection systems provide an optical efficiency of only ⅓ of that of three-panel projection systems because red (R), green (G), and blue (B) colors into which white light is separated are used sequentially. Hence, attempts have been made to increase the optical efficiency of single-panel projection systems.
Generally, in a single-panel projection system, light radiated from a white light source is separated into R, G, and B color beams using color filters, and the three color beams are sequentially transmitted to a light valve. The light valve operates and creates images according to the sequence of color beams received. As described above, a single-panel projection system processes color beams sequentially, therefore, the light efficiency is reduced to ⅓ of the light efficiency of a three-panel projection system. According to one color scrolling method designed to increase the optical efficiency of a single-panel projection system, white light is separated into R, G, and B color beams, and the three color beams are sent simultaneously to different locations on a light valve. Since an image cannot be produced until all of the R, G, and B color beams for each pixel reach the light valve, the color beams are moved at a constant speed using a color scrolling method.
In a single-panel scrolling projection system, as shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarization conversion system (PCS) 105 and is separated into R, G, and B color beams by first through fourth dichroic filters 109, 112, 139, and 122. To be more specific, the red beam R and the blue beam B, for example, are transmitted by the first dichroic filter 109 and advance along a first light path L1, while the green beam G is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the blue beam B on the first light path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path L1 and reflects the blue beam B along a third light path L3.
First through third prisms 114, 135 and 142 are disposed on the first through third light paths L1, L2, and L3, respectively. The light emitted from the light source 100 is separated into the R, G, and B beams, and they are then scrolled while passing through corresponding first through third prisms 114, 135, and 142. The first through third prisms 114, 135, and 142 rotate at a uniform speed such that R, G, and B color bars are scrolled. The G and B beams that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and thereby combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 122. The combined beam is transmitted to a light valve 130 via a polarization beam splitter (PBS) 127. The light valve 130 forms a picture.
A condensing lens 107 is disposed next to the PCS 105, and light path correction lenses 110, 117, 131, 137, and 145 are disposed along the first through third light paths L1, L2, and L3. Condensing lenses 120 and 140 are disposed between the second and fourth dichroic filters 112 and 122 and between the third and fourth dichroic filters 139 and 122, respectively. A focusing lens 124 and a polarizer 125 are disposed on the light path between the fourth dichroic filter 122 and the PBS 127. Light path changers, for example, mirrors 118 and 133, are disposed on the first and second light paths L1 and L2, respectively.
The periodic scrolling of the R, G, and B color bars due to rotation of the first through third prisms 114, 135, and 142 is illustrated in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 114, 135, and 142 corresponding to R, G, and B colors are rotated synchronously. As described above, as R, G, and B color bars circulate during one cycle, one frame of a color image is formed.
A color image, obtained by turning on or off the individual pixels of the light valve 130 according to an image signal, is magnified by a projection lens (not shown). Then, the magnified image is made incident on a screen.
First, second, and third slits 113, 134, and 143 are disposed in front of the first, second, and third prisms 114, 135, and 142, respectively, and control the divergence angle of incident light.
Since the conventional projection system uses different light paths for each color as described above, a light path correction lenses must be included for each color. Additionally, components are also required for recombining separated color beams Hence, the optical system becomes bulky, and the manufacture and assembly thereof is complicated, thus degrading the yield. In addition, three motors (not shown) for rotating the first, second, and third prisms 114, 135, and 142 generate a lot of noise during operation, and a projection system utilizing three motors is manufactured at a greater cost than a color wheel type projection system which utilizes a single motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. Since the conventional projection system must synchronize the light valve 130 with the three prisms 114, 135, and 142 in order to achieve scrolling, controlling the synchronization is not easy. Further, because the first, second, and third prisms 114, 135, and 142 are circularly rotated, the color scrolling speed is irregular, consequently deteriorating the quality of the resultant image.
The widths of the three color bars vary according to the width of each beam traveling along the first, second, and third light paths L1, L2, and L3. If the widths of the beams traveling along the first, second, and third light paths L1, L2, and L3 decrease, the R, G, and B color bars are narrowed such that black bars K are formed between adjacent color bars as illustrated in FIG. 3A. On the other hand, if the widths of the beams traveling along the first, second, and third light paths L1, L2, and L3 increase, the R, G, and B color bars are enlarged such that overlapping portions P are formed between adjacent color bars as illustrated in FIG. 3B.
When a liquid crystal display (LCD) is used as the light valve 130, the black bars K must be formed because consecutively processing image signals for color bars is difficult. More specifically, when color bars are consecutively scrolled on the LCD, image signals change every time the color bars are scrolled. However, consecutively processing the changing image signals is difficult. FIG. 3C illustrates R, G, and B color bars formed on the light valve 130 and the on-off states of the light valve 130 according to input image signals corresponding to the R, G, and B color bars. The time required to turn on image signals corresponding to the R, G, and B color bars is referred to as a rising time, and a rising area is indicated by S. The time required to turn off the image signals corresponding to the R, G, and B color bars is referred to as a falling time, and a falling area is indicated by T. The rising time and the falling time are required to turn on and off a group of image signals which change every time the R, G, and B color bars are scrolled. To allow for these rising and falling periods of time, black bars K must be formed between adjacent color bars.
As described above, systems which form a color image using a color scrolling technique form black bars by controlling the widths of the color bars.